A cascade converter system has advantages of high power density, high efficiency and easy modularization, which is expected to be widely used in occasions such as data centers and charging piles.
The cascade converter system may be operated in a single-phase or three-phase manner. Each phase is typically formed by cascading a plurality of converter modules. The converter modules may be connected in input-series-output-series, input-series-output-parallel, or other forms. For example, the cascade converter system may be used as a power electronic transformer, and each cascaded converter module may output a square wave or a step wave voltage. By superimposing voltages of these waveforms, it is possible to form a step wave voltage of more levels, to approach a sine wave voltage.
The cascade converter system typically improves its reliability by adding redundant converter modules. For example, when one of the cascaded converter modules fails, the faulty converter module is bypassed to allow it to quit operation, and other modules are still able to function normally, so that the entire system will not stop running.
However, the number of redundant converter modules in the cascade converter system is always limited. When all the redundant converter modules are bypassed due to faults and if still another converter module fails, the whole system will not be able to synthesize a qualified waveform and will quit operation due to the insufficient number of the overall converter modules.